php exec giving permission denied error

Dear All,
I have a java application which send sms. So on console it works fine no problem. The problem comes when I start to run from the web browser using the php exec method. Then I get error as "PortInUseException : java.lang.RuntimeException:
Error opening "/dev/ttyUSB0"
Permission denied".
I guess I must do some permission settings in linux any help please? Thank you.